Control of electrolytic coloring of chromium-containing alloys

ABSTRACT

A PROCESS FOR COLORING STAINLESS STEEL AND OTHER CHROMIUM-CONTAINING ALLOYS COMPRISING ANODICALLY TREATING THE METAL TO BE COLORED IN AN ACID CHROMATE ELECTROLYTE AND REMOVING THE METAL FROM THE ELECTROLYTE WHEN THE METAL ELECTROLYTE POTENTIAL RELATIVE TO A REFERENCE ELECTRODE REACHES A PREDETERMINED VALUE.

United States Patent Int. orczsn 11/02 US. Cl. 204-56 R 8 Claims ABSTRACT OF THE DISCLOSURE A process for coloring stainless steel and other chromium-containing alloys comprising anodically treating the metal to be colored in an acid chromate electrolyte and removing the metal from the electrolyte when the metalelectrolyte potential relative to a reference electrode reaches a predetermined value.

This invention relates to the coloring of stainless steel and other corrosion-resistant chromium-containing alloys having a base of one or more of iron, cobalt and nickel by means of aqueous solutions consisting essentially of chromic acid and sulfuric acid, with or without other additions such as manganous sulphate.

It is known for example from the work of C. E. Naylor (Plating, 1950, pages 153 if) that the formation of colored films on stainless steel surfaces by immersing the steel in a solution of sulfuric acid and sodium dichromate can be accelerated by passing a direct current between the steel as anode and a lead cathode, which may conveniently be the lead lining of the tank containing the solution. As the electrolysis continuous the color obtained progresses through the sequence brown, blue, yellow, reddish-purple, deep purple and green. The color obtained depends on the duration of the electrolysis, but attempts to control the color of the film by this means have not proved satisfactory, since the time required to develop a given color varies markedly with the surface condition of the steel.

It is an object of the present invention to provide a novel controlled process for electrolytic coloring of stainless steel and other corrosion-resistant chromium containing alloys.

Other objects and advantages will become apparent from the following:

The present invention is based on the discovery that a more accurate indication of the progress of the coloring is provided by monitoring the potential of the steel or other metal being colored relative to a reference electrode while direct current is passing through a cell consisting of the steel or other metal being colored as anode, a solution containing chromic acid or a soluble salt of chromic acid (e.g. a chromate or dichromate) and sulfuric acid i.e., the solution contains chromate or dichromate ion or allied ionization products and sulfuric acid in water, and a cathode consisting of lead, platinum or other suitable metal. If the solution is contained in a lead-lined tank, the tank lining may conveniently be used as cathode.

The reference electrode is conveniently an inert metal or alloy immersed in the coloring solution, but other standard electrodes, e.g., a saturated calomel or mercurous sulphate electrode having a bridge that is chemically stable in contact with the coloring solution, may if desired be used.

In particular, we make use of the fact that a relationship exists between the color of the film formed on the surface of the metal and the potential of the metal, relative to the reference electrode. This relationship may be both predetermined and used in various ways.

According to the invention, electrolysis is discontinued when the potential reaches a predetermined value. In the simplest way of carrying out the invention, the potential corresponding to the desired color is first determined during the coloring of a similar specimen of metal by simple immersion in a coloring solution of the same composition at the same temperature as is to be used for the electrolysis. Advantageously a potentiostat is then used to maintain the potential of a further specimen of the steel or other metal being colored at a suitable constant value, relative to the reference electrode. The potentiostat performs this function by supplying sufiicient current flow through a cell consisting of the steel and a suitable counter-electrode, e.g., platinum or lead, to maintain a constant potential on the steel relative to the reference electrode. The potentiostat is set to maintain the specimen at the potential which is known, from prior calibration of the color-potential relationship, to correspond to the desired color. The anodic current density on the material being colored is monitored and when it falls to zero the formation of the film of desired color is complete.

The actual potential corresponding to a particular color depends both on the composition and the surface condition of the metal as well as on the solution composition and temperature, and this simple method is only satisfactory if a series of substantially identical specimens is to be treated. We prefer therefore to make use of the further discovery that for a given combination of metal, coloring solution and temperature the difference between the potential at which the film responsible for the appearance of color begins to form (the start potential) and that at which a given color is obtained is constant.

The start of film formation is indicated by a minimum in the rate of change of potential with time (dE/dt), and this point may be readily determined from an automatically recorded trace of potential (E) as a function of time (t). In most cases, and in particular with Type 304 stainless steel, dE/dt falls to zero, i.e., the potential remains constant for a short period and the potential at which this happens is the start potential. However, dE/dt does not always fall exactly to zero, in which case the potential-time curve exhibits a point of infiexion at which dE/dt passes through a minimum; the potential at such a point of inflexion is then taken as the start potential.

In this way of carrying out the invention, the difference between the start potential and the potential for the desired color is first determined for the system. The start potential is then determined for the specimen being colored and electrolysis is continued to the end-point determined by the difference in potential corresponding to the desired color and then stopped.

The difference between the start potential and the potential corresponding to a particular color is found to be slightly less when measured with an anodic current flowing through the metal being colored than under simple immersion conditions, and the calibration should therefore preferably be carried out with an impressed current. Also, the value of the start potential increases as the impressed current increases.

This procedure may be carried out in the following way, with the use of a potentiostat. The potentiostat is set to maintain the metal to be colored at any suitable potential which will produce a conveniently measurable anodic current. The anodic current is found to rise to a maximum within 1 to 2 minutes. At this point the potentiostat is disconnected and the potential of the metal, relative to the reference electrode, is measured. This potential corresponds to the start potential. The potentiostat is then reconnected and the potential set at the value corresponding to the potential for formation of a film of the desired color. The current flowing through the cell is monitored and when the current falls to zero, the specimen has attained the desired color. If the electrolysis is continued beyond this point, the current flowing through the material being colored becomes cathodic and coloring continues through the normal sequence of colors, although at a reduced rate.

Althernatively, use may be made of a galvanostat (a device supplying constant current) to maintain a constant cell current between stainless steel or other metal being colored as anode and a suitable cathode, e.g., platinum or lead. Providing the anodic current density is not excessive, e.g., below about 0.25 A/dm. a potential vs. time curve is observed having the characteristics described above. A film of the desired color is obtained on the anode material by maintaining the cell current constant until the potential of the anode, measured as described above, attains the required value and then terminating the treatment. If the current density on the anode is excessive, i.e., much greater than 0.25 A/dmf it is difiicult to observe the shape of the potential vs. time curve because the potential of the steel rises rapidly to a high value, and the material will then be etched.

Generally speaking it has been observed that applied potential differences across the cell in the range 1.9 to 2.8 volts generate anodic current densities in the range 0.025 to 0.25 A/dmfi, and that application of such anodic current densities halves the time required to produce the colored film.

Aqueous electrolytes useful in the present invention have compositions as follows:

Range, Preferred, g./l. g./l.

200-400 240-300 350-700 450-550 Balance Balance It is advantageous to operate the controlled electrolytic coloring process of the present invention at a temperature in the range of about 60 to about 80 C.

Colored coatings produced by direct current electrolysis in accordance with the invention can be hardened and rendered more abrasion-resistant by the method described in US. patent applications Ser. Nos. 114,357, 252,459 and 301,810, and the invention includes the application of these hardening treatments to the colored coatings.

Some examples will now be given.

A speciment of Type 304 stainless steel was colored by immersion in an aqueous solution of 2.5 M CrO and 5 M H SO at 70 C. The potential of the steel against a saturated calomel electrode was monitored, and the potential change from the plateau observed at the start potential to the development of a blue color was found to be 7 mv.

Further specimens of this steel were then treated in the same solution as follows.

EXAMPLE I A steel specimen was immersed in the solution at the same temperature with a platinum counter-electrode, and a potentiostat was set to maintain the potential of the specimen at a value which created a conveniently measurable anodic current in the specimen/counter-electrode cell. This current reached a maximum after 30 sec. at which time the potentiostat was switched off and the potential of the specimen relative to the reference electrode was measured as being +1110 mv. This value was taken as the start potential for color control. The potentiostat was reset to maintain the specimen potential at +1117 mv. (i.e., 7 mv. higher than the start potential) and the anodic cell current monitored. When the cell current fell to zero the specimen was removed and was found, as expected, to be co ored blue.

.4 EXAMPLE II A steel specimen was immersed in the same solution at C. as anode with a platinum electrode as cathode, and a galvanostat was used to maintain a constant current density of 0.025 A/dm. on the specimen. The potential of the specimen relative to the reference electrode was monitored. The specimen potential rose quickly to a value of +1085 (SCE) at which it remained for 4 min. This value was taken as the start potential. The specimen potential then commenced to rise again. When the specimen potential had risen 7 mv. above the start potential the specimen was removed, and was found, as expected, to be colored blue.

EXAMPLE III A steel specimen was immersed in the same solution at 70 C. as anode with a lead cathode and a direct current power supply was used to set a potential of 2.1 v. across the specimen/lead cell. The potential of the specimen relative to the reference electrode was monitored.

The specimen potential rose quickly to +1050 mv. (SCE) at which value it remained for 1 min. This value was taken as the start potential. The specimen potential then commenced to rise again. When the specimen potential had risen 7 mv. above the start potential the specimen was removed, and was found, as expected, to be colored blue.

Corrosion resistant, chromium-containing alloys other than steel which can be treated in accordance with the present invention usually contain at least about 12.5% by weight of chromium and include iron-based nickelchromium-m0lybdenum alloys such as an alloy containing 37% nickel, 18% chromium, 5% molybdenum, 1.2% titanium and 1.2% aluminum; cobalt-based alloys such as that containing 21% chromium, 21% nickel and 13% molybdenum; and nickel chromium alloys, such as an alloy containing 30% chromium and 1% titanium, the remainder being nickel. For purposes of this specification and claims, stainless steels include iron-base alloys containing greater than about 11% and up to about 30% chromium.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process for coloring chromium-containing metal comprising anodically treating said chromium-containing metal in an aqueous solution of chromic acid or ions derived from a water-soluble chromate or dichromate and sulfuric acid, causing cessation of flow of direct current responsible for the anodic treatment when the potential of said chromium-containing metal in said aqueous solution measured against a reference electrode reaches a pre-determined value associated with a particular color and thereupon removing the chromium-containing metal from said aqueous solution.

2. A process as in claim 1 wherein said chromiumcontaining metal in said aqueous solution is maintained relative to said reference electrode at a constant potential, associated with a particular color by means of an electric current induced in the cathode-electrolyte anodepotential source circuit, the current automatically diminishing to zero when formation of the film of said particular color is complete.

3. A process as in claim 2 wherein said chromiumcontaining metal is stainless steel.

4. A process as in claim 3 wherein said chromiumcontaining metal is stainless steel.

5. A process as in claim 3 wherein said anode current density is about 0.025 to about 0.25 am ere/(1111.

6. A process as in claim 1 wherein said chromiumcontaining metal in said aqueous solution is part of a cathode-electrolyte-anode-potential source circuit in which an essentially constant current sufiicient to provide an anode current density of up to about 0.25 ampere/dm. is caused to flow until the potential of said chromium-containing metal measured against said reference electrode reaches said' predetermined value.

7. A process as in claim 1 wherein the predetermined potential value is a value arrived at by measuring the inflexion potential of said chromium-containing metal at the start of coloring and adjusting said infiexiou potential by an increment associated with desired coloration.

8. A process as in claim 1 wherein said chromiumcontaining metal is stainless steel.

References Cited GERALD L. KAPLAN, Primary Examiner 10 R. L. ANDREWS, Assistant Examiner 

